Condenser



April s, 1930. M. H. BENNr-TT 1,753,193

CONDENSER Filed Oct. 23, 1926 3 Sheets-Shee1.l l

|.5OO 456 Losa April 8, 1939. M. H. BENNETT I 1,753,193

CONDENSER Filed Oct. 23, 1926 3 Sheets-Sheet 2 LSH 757" |.523 19| April 8, 1930. M. H. 'BENNETT 1,753,193

CONDENSER Filed Oct. 25, 1926 5 Sheets-Sheet 5 m cd v Q9 9 j (l1 01 lig ,7, I i) m 0, oo

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5 no \5 2o 25 30 3540 45 5o 55 e5 '70 75 5o e5 SQ 95| nvemtoz Patented Apr. 8, 1930 UNITED STATT-:s

PATENT OFFICE MORRIS H. BENNETT, OF WATERBURY, CONNECTICUT, ASSIGNOR TO'SCOVILL MANU- FACTURING COMPANY, OF WATERBURY, CONNECTICUT, A CORPORATION OF CON NECTIGTT coNDENsEn A Application led October 23, 1926. Serial No. 143,573.

The present invention relates to improvements in variable condensers and particularly to condensers suitable for use in radio receiving sets.

It has been common in the art heretofore to provide variable condensers in which the plates were so formed that the changes of capacity in the circuit tuned by such acondenser, considered in relationto the movements which produced the changes, would take pla-ce according' to some given mathematical law, as for example, the law of direct proportion or so-called straight line capacity law; the logarithmic or so-called straight line frequency law, and the square or so-called straight line wave length law. Owing to the fact that by international agreement and the regulations of the government, a certain range of frequencies has been allotted to broadcasting stations in the United States, so that, barring some duplications, the respective stations transmit at different fre quencies varying from each other by 10 (ten) kilocycles, there has recently been a general adoption of the logarithmic law for such variable condensers because when included in the usual receiving circuits Where the apparent inductance increases with increase of frequency, such condensers distribute the stations of the United States substantially uniformly over the dial which is turned to move the variable member of the condenser.

A variable candenser of this so-called straight line frequency type has a number of disadvantages. For example, the movable plates have a very short radius at what may be called the minimum setting and a relatively long radius at the maximum setting which is about 180 from the minimum setting. Therefore, the weight of the set of movable plates is greatly unbalanced, with the result that the rotor spindle is stressed laterally in its bearings. Furthermore, when such a condenser is mounted with the plates vertical there is a tendency for the movable plates to drop down to a position where the greatest radii of the plates extend vertically downward from the axis of'ro'tation. This tendency is overcome by considerably increasing the friction of the spindle and bearings,

smoothly from one position to another. Furthermore, condensers of this typeoccupy more space in the receiving apparatus, be-

' cause it is necessary to provide more space for the plates to rotate in.

Applicant has discovered that for the ordinary receiving set which is to remain at any given locality the so-called straight line frequency variable condenser is based on an erroneous assumption. A receiving set of the usual type located at any given place will not receive from all the broadcasting stations in the. United States. It may be sensitive enough to receive from the most distant high powdered stations, but it will not receive from distant low owered stations. Therefore, in such a receiving apparatus, with such a condenser, the stations which can be effectively received are not distributed uniforml over the dial of the condenser, there being re atively large intervals on the dial where no stations are received, These intervals of nonw reception differ, of course, according to the locality in which the receiving set is placed, but generally do not fall in that part of the dial where the capacity of the condenser is high and the received frequency low because the high power transmitting stations, which are generally expected to be received at all localities by a satisfactory receiving set, work at the lower range of frequencies where the receiving set is using the larger capacities of the condenser.

I have found that a variable condenser can be made, which is satisfactory for receiving sets to be located y,anywhere in the United States and which will overcome the chief disadvantages of the so-called straight line frequency variable condensers while at the same time giving a better distribution on the dial of stations actually received. A .condenser made according to my invention does not have plates formed to give capacity changes following one mathematical law throughout the working range, but the plates are so formed that the capacity changes will follow different mathematical laws over different angular sections of the plate. In carrying the invention into practice, I make each" movable plate of such form that from the point of minimum capacity toward the point of maximum capacity, there is a zone in which the changes of capacity will be such that in operation in the receiver, the uniform graduations ofthe dial which operates the condenser will indicate uniform vdecreases of kilocycles, and that from the point of maximum capacity backward toward the point of minimum capacity there is a zone in which the changes of capacity are such that in op'- eration in the receiver, the dial graduations will indicate either'wave lengths or will be proportional to the capacity of the condenser.

Inother words, a variable condenser embodying my invention is so constructed that when included in the receiver circuit it will be a straight line frequency condenser at that end where the capacities are small and either a straight line capacity condenser or a straightline wave length condenser at the opposite end where the capacities are large. These two zones may constitute the whole range of the condenser, but in practice it generally occurs that there is a short angular zone intermediate the said two zones which is determined empirically to blend the two zones, that is, to give a gradual change of capacity such that there will be no sudden change from one law of change to t-he other.

Over this small zone, the change of capacity will not differ much from what it would be if the plates at this part were formed to follow either of the two laws of change embodied in t-he two end zones. It has been found in practice, that for a rotary plate intended to turn through 180 of arc, the intermediate or blending Zone should not substantially exceed 18 of are., In many cases it may be much less, this depending upon the kind of receiver circuit with which the condenser is to work. In some cases the intermediate blending zone may be omitted, as, for example, where the curvatures of the two zones meet each other at a point of' substantiallyl common tangency.

The accompan ing drawings illustrate some examples o plates for variable condensers made in accordance with this invention.

In the"drawingsy -Figure 1 illustrates a single rotor plate of the type to be used with a receiver circuit having relatively low distributive capacit-y;

Figure 2 is a view similar to Fig. 1 of a plate to be used in a condenser working with a receiver having a pair of circuits in which the distributive capacities are practically balanced;

Figure 3 is a view similar to 1 of a plate to be used in a condenser Working in a receiver circuit having a relatively high distributive capacity;

Figures 4, 5 and 6 are views illustrating stator plates suitable for use with the rotor plates of Figs. 1, 2 and 3 respectively;

Figure 7 is a diagram showing the characteristic curve of a 15 plate condenser embodying the invention.

The plate shown in Fig. 1 is particularly suitable for a tuned radio frequency receiving set having a transformer with a relatively low number of turns and having its oscillations controlled by a potentiometer whereby a resistance may be introduced into the circuit. In such a circuit the distributive capacity is relatively low and the variable condenser plates are dimensioned to give the desired result. Each plate is so formed that when used in a circuit of low distributive capacity, such, for example as the circuit above described, uniform differences of movement of the dial, from the minimum capacity of the condenser through an angle of 90 will give uniform changes of frequency in the circuit containing the condenser. In other words, the first 90 of the condenser follows the mathematical law of straight line frequency.

The last 72 of the condenser plate, that is the zone which terminates at the maximum capacity of the plate and which in this example extends from 108 to 180, is made with a curve of uniform radius. Over this zone the condenser follows a straight line capacity law. When the condenser is being used in this zone, the stations are not distributed exactly uniformly according to their frequencies, but the stations will be spread apart on the operating dial. The difference, however, is not great enough to be disadvantageous.

In the intermediate zone of the condenser, that is between 90 and 108 the formation of the plates is such as to give a gradual change of capacity from the largest radius of the Zone which follows the straight line frequency law to the uniform radius of the zone which follows the straight line capacity law. The curve of the margin of the plate over this intermediate zone may be determined empirically, but it will be found that if the radius through the center of this zone be taken as somewhat greater than the arithmetical means of the two radii bounding this zone, the results will be satisfactory. In the example shown in Fig. 1, the central radius through this intermediate zone is 1.270 inches, while the bounding radii are 1.232 and 1.300

inches, respectively. The arithmetical mean circuits in which the distributive capacities are practically balanced. One form of receiving'apparatus embodying this principle isknown on the market as an Isofarad receiver. A circuit system of this general type requires some changes of the shape of the entrance part of the plate in order to get straight line frequency from about() to 108 of angular movement. s

As will be seen from the dimensions in Fig 2, the radius at the entrance portion of the rotor is somewhat smaller absolutely than with the plates shown in Fig. 1, while at the same time the succeeding radii increase in value more rapidly than with said plate in Fig. 1. The first 108 of a plate having this ratio of dimensions, gives straight line frequency when-used with circuits having no material distributive capacity, as in the circuits of thesaid Isofarad receiver, where the distributive capacities are balanced, or practically so. The last 63 of the plate, that is, the portion having the larger radius may be made of constant radius, as indicated in the drawing, Fig. 2, thereby giving straight line capacity at this part. There is an intermediate interval of the plate between the first 108 and the last 63 which is curved empirically to blend with the straight line frequency curve and with the straight line capacity curve. This blended portion comprises 9 and the plate is so shaped at its edge that the capacity changes gradually. This blended portion may be determined in the same manner as the blended portion described'inconnection with the plate in Fig. 1. In Fig. 3 is illustrated a plate for a condenser to be used with a circuit having relatively high distributive capacity so as to give straight line frequency characteristics through an angle of about 108 measured from the smallest radius of the plate. The last part of the dial has a -constant radius as indicated in Fig. 3 over the last 54. However, owing tothe arrangement of the intermediate part of the plate (amounting to about 18) between the first 108 of the plate and the last 54, the resulting action of the plate as it is turned into the stator is to give a close approximation to a straight line wave length characteristic from the angle of 108 to the angle of 180. The intermediate portion which serves to blend the'straight line frequency portion of the plate with the last portion of the plate is so dimensioned that the intermediate radius is about equal to the arithmetical mean of the two radii which bound this blending zone. Hence in practice, it has been found, with a condenser having plates designed as shown in said Fig. 3 and used in a circuit having relatively high distributive capacity, that the,last 7 2 of the plate gives such an approach tostraight line wave length characteristws that 1t may be considered as entirely equivalent to a plate having that portion laid out by the straight line wave .length formula.

' Figs. 4, 5 and 6 show the forms of the respective stator plates employed in condensers using the plates of Figs. 1, 2 and 3, respectively, The purposev of making the stator plates in the forms shown in these Figs. 4, 5 and 6 instead of substahtially semi-circular, as is usually done, is to reduce the amount of material employed and at the same time to make the condenser more compact. v

Fig. 7 is a diagram showing the relationship otcapacities to dial readings, the curve being the characteristic curve ofy a condenser having rotor plates such as shown in Fig. 1, where the first 108 of the curve has a straight line frequency characteristic, andthe last 7 2 of the curve has substantially a straight line capacity characteristic.

It will be understood that the rotor plates are mounted 0n a revoluble spindleV in the way customary in variable condensers now in general use, and that any number of plates may be used according to the desired maximum and minimum of the condenser.

It will be understood, of course, that the measurements of radii stated in the drawings are merely given as examples 'of specific condensers. The invention is not limited to thesey denser a characteristic according to some other law over another part of its angular movement, where the capacities are large,

said compound curve forming a. plate area substantially equal to the are'a formed by the equivalent straight line frequency curve for the angular and the capacity ranges of the condenser.

Q. A variable. electrical condenser having stator plates and rotor plates arranged to cooperate with the stator plates, said rotor plates each having-its periphery formed to a compound curve such as to give the condenser substantially a straight line frequency characteristic over that portion of the plate where the capacities are small and to give the condensera straight line capacity characteristic over that part of its angular movement where the introduced part ofthe rotor plate has the relatively large capacities, said compound curve forming a plate `area substantially equal to the area formed by the equivalent straight line frequency curve for the ngular and the capacity ranges of the conenser.

3. A variable electric condenser having stator plates and rotor plates arranged to cooperate with the stator plates, each of said rotor plates having its peripheral edge formed to a compound curve such as to give to the condenser, When in an appropriate circuit of a raido receiver, a straight line frequency characteristic over that portion of the movement of the condenser Where its capacities areprelatively small and a characteristic `following some law other than straight line frequency over that portion of the movement of the condenser Where its capacities are relatively large, said compound curve forming a plate area' substantially equal to the area formed'by the equivalent straight line frequency curve for the angular and the capacity ranges of the condenser.

4. A variable electrical condenser having stator plates and rotor plates movable relative to each otherto vary the capacity of the condenser according to variation of the area of the overlapping portions of the respective rotor and stator plates While maintaining substantially uniform dielectric distances of the plates, said plates having a compound curve such that the relation of the change of capacity to relative movement of the rotor and stator plates will, when the condenser is included ina radio circuit, give substantially a straight line frequency characteristic over at least half of the relative movement and Where the capacities are relatively small, and substantially a straight line capacity characteristic over substantially all of the remaining movement of the condenser Where the capaclties are relatively large, said compound curve forming a plate area substantially equal to the area formed by the equivalent straight line frequency curve for the angular and the capacity ranges of the condenser.'

In testimony whereof, I have hereunto set my hand.

' 'MORRIS H. BENNETT 

